Methanol Production Process and System

ABSTRACT

A process and a system are disclosed for producing methanol from synthesis gas. The synthesis gas is a stream containing H 2 , CO, and CO 2  that is created using a nitrogen containing oxidant stream, such as air. The synthesis gas is then reacted through a conventional reactor system to create methanol. Unreacted synthesis gas is recycled back through the reactor system. The disclosed methanol production process can be mounted and operated on a seagoing vessel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. patent application Ser. No.11/655,475 filed Jan. 19, 2007, which in turn claims priority to U.S.Provisional Application No. 60/809,260 filed May 30, 2006, both of theseapplications are incorporated herein in their entirety for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process and system for production ofmethanol. More specifically, the invention includes a process and asystem for producing methanol from combining a synthesis gas containinginerts, notably, nitrogen with a hydrogen-rich gas stream, andsubsequently converting the hydrogen-rich synthesis gas stream tomethanol, or other hydrocarbon chemicals. Even more specifically, theinvention relates to such processes and systems capable of being mountedand operated on seagoing vessels, as well as on land.

2. Description of the Related Art

The evolution of methanol synthesis started at the beginning of the 20thcentury and reached commercialization in the mid 1920s. The reactantsfor methanol synthesis were H₂, CO, and CO₂, and this mixture was named“synthesis gas” or “syn gas.” A stoichiometric mixture when passedthrough a catalyst bed, only reacted 12% of these reactants because thereaction was stopped by reaching equilibrium with the methanol generatedby the reaction. The industry adopted the use of a recycle system, knownas a “methanol loop,” as a solution. The reacted gases from the catalystbed were cooled, methanol condensed out, and the remaining gasesre-circulated through the inlet to the catalyst bed. This system had theadvantage of absorbing the exothermic heat generated at the catalystsite and carrying the heat to external heat exchangers. Up to 95% of thereactants were converted to methanol by this technique. This has beenused in virtually all development of methanol synthesis during the last80 years.

There are disadvantages of this system. The methanol loop placed arestriction on the purity of the syn gas. Consequently, inerts fed intothe methanol loop with the syn gas had to be purged from the loop justafter the methanol was condensed out. The purged inerts were intimatelymixed with the valuable reactants, which were lost when purged alongwith the inerts. The prior art solution to this problem was to reducethe amount of inerts to as little as possible during syn gaspreparation.

Modern practice in high capacity plants is to use an autothermalreformer (“ATR”) to prepare the syn gas. The device mixes an oxygencontaining gas stream with a natural gas stream to partially oxidize thenatural gas in the top of the reformer vessel. The lower portion of thereformer vessel contains a catalyst, which brings the oxidized gasesinto chemical equilibrium. The major ingredient in natural gas ismethane, which is converted to syn gas and H₂O.

Use of an ATR is also problematic because inerts are introduced into thenatural gas stream along with the oxygen. To combat this problem, acryogenic air separation (“C-ASU”) was utilized to produce oxygen withthe lowest possible inerts, generally between about 1% to 5%. Themajority of the inerts were N₂ and Ar. Currently, there are no viablealternative processes for producing oxygen at large capacity and purityin this range. Cryogenic air separation is a difficult process tooperate, has high maintenance and has a history of catastrophicexplosions. Additionally, the CH₄ in the reformer vessel outlet acts asan inert in the methanol loop. Therefore, furnaces and reformers wereoperated at temperatures on the extreme upper limits of metal andceramic materials to minimize the CH₄ remaining in the syn gas. Forthese reasons, the designs were expensive and consumed up to 80% of theoverall plant energy.

Over the last 80 years, this approach has taken its toll in capitalcosts of the synthesis gas production portion of methanol plants.

COST BREAKDOWN BY PLANT SECTION Syngas preparation  60% Methanolsynthesis loop 10 Methanol distillation 10 Utilities 20 100%

Prior art methanol production processes include those manufactured andsold by Lurgi AG of Frankfurt, Germany. Such prior art Lurgi systemshave been disclosed, for example, in European Patent 0790226 B1, U.S.Pat. No. 5,631,302 and U.S. Pat. No. 5,827,901, which are herebyincorporated by reference in their entireties. Similar systems utilizinga methanol loop include those used by ICI and Holder-Topsoe, includingthose systems described in U.S. Pat. Nos. 6,387,963 6,881,759,6,730,285, 6,191,175, 5,937,631 and 5,262,443, and United Kingdom PatentNos.: 1,159,035 and 1,272,798, which are all hereby incorporated byreference in their entireties.

Synthesis gas has also been manufactured from oxidant streams high innitrogen, such as air. Such processes have used separation processes,such as semi-permeable membrane technology, to separate air streams intohigh oxygen content streams and high nitrogen content streams. The highoxygen content streams were then reacted with natural gas to createsynthesis gas, which was then converted to methanol.

Special reaction systems had to be developed because the high nitrogencontent in the synthesis gas stream created problems in conventionalmethanol production processes by limiting the yield and theeffectiveness of the methanol reactors. Such processes are disclosed inU.S. Pat. Nos. 5,472,986 and 7,019,039, which are hereby incorporated byreference in their entireties. These patents are assigned to StarchemTechnologies, Inc. and the methanol production processes describedtherein are generally referred to herein as the “Starchem system.” Inthe Starchem system, a reactor recycle stream (methanol loop) was notused because of problems associated with the high nitrogen content. Assuch, a series of single pass reactors were required.

Similarly, European Patent Application 0 261 771 proposed the use of airfor production of a high nitrogen content synthesis gas which,thereafter, would be processed through a series of plug flow methanolreactors with interstage removal of methanol and water. As such, aseries of single pass reactors were required, just as in the Starchemsystem.

The ATR and the methanol loop are not compatible without modifications.This can be explained in terms of the stoichiometric number (“N_(s)”)defined as N_(s)=(H₂−CO₂)/(CO+CO₂).

N_(s) is commonly used as a measure of how syn gas will perform in themethanol loop. A number greater than 2 indicates an excess of hydrogenover that required for conversion of all the carbon to methanol. Anumber less than 2 indicates a hydrogen deficiency. The methanol loopmay become in-operable when deficient in hydrogen. Make up gas (“MUG”)is the name of the gas injected into the methanol loop. Experience hasshown that a MUG with N_(s)=2.05 produces the most efficient and lowestcapital cost methanol loop design.

A characteristic of an ATR is the reformed syngas has an N_(s) of about1.75. The traditional approach has been to add hydrogen to the effluentof the ATR to increase the N_(s) of the MUG stream to about 2.05. Thesource of hydrogen has been from fired steam methane reforming or fromrefineries. More recently, the N_(s) has been increased by rejecting CO₂from the gas mixture as in Starchem's U.S. Pat. No. 7,019,039 for aseries of single pass reactors.

It would be desirable to utilize prior art methanol production systemsthat include a methanol loop, with a synthesis gas produced throughpartial oxidation of natural gas using an air stream, such as theStarchem system. U.K. Patent Application 2,237,287A and AustralianPatent AU-B-6459390 (“the AUS Process”) describe the use of a synthesisgas formed from an oxygen enriched gas stream for the partial oxidationof natural gas and the use of a methanol loop reactor system formethanol production. In the AUS Process, a portion of the synthesis gasis not subject to the methanol synthesis loop. Rather, the synthesis gasis split into two distinct streams, stream “A” and stream “B,” uponleaving the ATR. The “A” stream is diverted to a water gas shiftreactor, converting CO to H₂, and then through a pressure swing absorber(“PSA”) to extract the H₂. The H₂ that has been extracted joins the “B”stream and the combined flow is a hydrogen enhanced syn gas. This methodis less than desirable because of the need for additional equipment forthe extraction of the H₂ and the diversion of some of the synthesis gas,which results in a reduction in potential methanol production.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features, and advantages of the presentinvention will become apparent upon further consideration of thefollowing detailed drawing figures, in which:

FIG. 1 shows an example of a simple process flow diagram in accordancewith the present invention;

FIG. 2 shows an example of a simple process flow diagram in accordancewith the present invention, including the preparation of syn gas fromenriched air and natural gas; and

FIGS. 3 and 4 are a graph and table that illustrate the results of thecomparative analysis of the AUS Process with the methanol productionprocesses of the invention.

BRIEF SUMMARY OF THE INVENTION

The present invention is a process and a system for creating methanolfrom synthesis gas. The synthesis gas is a stream containing H₂, CO, N₂and CO₂, generated from a N₂ containing oxygen stream, such as air. Thesynthesis gas stream is reacted through a methanol loop system to createmethanol. A specific embodiment of the present invention combines theStarchem system for creating synthesis gas, utilizing an air stream withthe conventional methanol loop system for reacting synthesis gas tocreate a methanol stream, while avoiding prior art concerns ofefficiency and methanol yield resulting from the presence of nitrogen inthe system. Another specific embodiment of the present invention is amethanol production process such as described herein, which can bemounted and operated on a seagoing vessel, as well as on land.

In one aspect, the invention provides a process for producing methanol,comprising the steps of providing a synthesis gas stream comprising H₂,CO, and CO₂, and N₂, wherein the synthesis gas stream comprises at leastabout 6 mole % N₂ and wherein the synthesis gas stream comprises H₂, CO,and CO₂ in a ratio of (H₂−CO₂)/(CO₂+CO), which is less than about 1.80.The synthesis gas stream is then combined with a hydrogen-rich gasstream to form a makeup gas stream, which is then combined with arecycle gas stream to produce a reactor feed stream. The reactor feedstream is introduced into a reactor system containing a methanolconversion catalyst, wherein a portion of the reactor feed stream isconverted to methanol. A reacted gas stream is withdrawn from thereactor system and the reacted gas stream is separated into a crudemethanol product stream and a gas stream. The gas stream is split into arecycle gas stream and a purge gas stream. The recycle gas stream ismixed with the makeup gas stream to form the reactor feed stream. Thepurge gas stream is then separated into a fuel gas stream comprising CH₄and N₂ and a hydrogen-rich stream comprising H₂, and the hydrogen-richstream is mixed into the synthesis gas stream.

In another aspect of the invention, a similar process for converting anatural gas to methanol is provided. The process includes providing anair stream having an oxygen content less than about 22% and enhancingthe oxygen content of the air stream to between about 28% and 94%oxygen, thereby creating an enhanced oxygen stream. A natural gas streamcomprising methane is then provided and the natural gas stream ispartially oxidized in an autothermal reformer using the enhanced oxygenstream to create a synthesis gas stream comprising H₂, CO, and CO₂ andN₂. The synthesis gas stream is then combined with a hydrogen-rich gasstream to form a makeup gas stream, wherein the synthesis gas streamcomposition remains substantially unchanged after exiting theautothermal reformer until combined with the hydrogen-rich gas stream.The makeup gas stream is combined with a recycle gas stream to produce areactor feed stream, which is introduced into a reactor systemcontaining a methanol conversion catalyst; wherein a portion of thereactor feed stream is converted to methanol. A reacted gas stream isthen withdrawn from the reactor system and separated into a crudemethanol product stream and a gas stream. The gas stream is split into arecycle gas stream and a purge gas stream. The recycle gas stream iscombined with the makeup gas stream to form the reactor feed stream. Thepurge gas stream is separated into a fuel gas stream comprising CH₄ andN₂ and a hydrogen-rich stream comprising H₂, and the hydrogen-richstream is mixed into the synthesis gas stream.

In yet another aspect of the invention, a process is provided forproducing methanol that comprises the steps of providing an air streamhaving an oxygen content less than about 22% and enhancing the oxygencontent of the air stream to between about 28% and 94% oxygen, therebycreating an enhanced oxygen stream. A natural gas stream is providedthat comprises methane. The natural gas is partially oxidized in anautothermal reformer using the enhanced oxygen stream to create asynthesis gas stream comprising H₂, CO, and CO₂, and N₂. At least about90% of the synthesis gas stream is then combined with a hydrogen-richgas stream to form a makeup gas stream, which is combined with a recyclegas stream to produce a reactor feed stream. The reactor feed stream isintroduced into a reactor system containing a methanol conversioncatalyst, wherein a portion of the reactor feed stream is converted tomethanol. A reacted gas stream is withdrawn from the reactor system,which is separated into a crude methanol product stream and a gasstream. The gas stream is split into a recycle gas stream and a purgegas stream. The recycle gas stream is then mixed with the makeup gasstream to form the reactor feed stream. The purge gas stream isseparated into a fuel gas stream comprising CH₄ and N₂ and ahydrogen-rich stream comprising H₂, and the hydrogen-rich stream ismixed into the synthesis gas stream.

In a further aspect of the invention, a process is provided forproducing methanol, which comprises the steps of providing a synthesisgas stream comprising H₂, CO, and CO₂ and N₂, wherein the synthesis gasstream comprises at least about 16 mole % N₂ and wherein the synthesisgas stream comprises H₂, CO, and CO₂ in a ratio of (H₂−CO₂)/(CO₂+CO) ofabout 1.73. The synthesis gas stream is combined with a hydrogen-richgas stream to form a makeup gas stream, which is combined with a recyclegas stream to produce a reactor feed stream. The reactor feed stream isthen introduced into a reactor system containing a methanol conversioncatalyst, wherein a portion of the reactor feed stream is converted tomethanol. A reacted gas stream is withdrawn from the reactor system andseparated into a crude methanol product stream and a gas stream. The gasstream is then split into a recycle gas stream and a purge gas stream.The recycle gas stream is mixed with the makeup gas stream to form thereactor feed stream. The purge gas stream is separated into a fuel gasstream comprising CH₄ and N₂ and a hydrogen-rich stream comprising H₂,and the hydrogen-rich stream is mixed into the synthesis gas stream.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, the Starchem system for creating synthesis gasfrom the oxidation of natural gas using an air or otheroxygen-containing stream is combined with a traditional methanol loopsystem. The present invention overcomes prior art concerns of usingnitrogen containing synthesis gas in the traditional methanol loopsystem.

Starchem systems such as those disclosed in U.S. Pat. No. 5,472,986 andU.S. Pat. No. 7,019,039 B1 (“Starchem Patents”) include processes forcreating synthesis gas using an air stream, or other oxygen and nitrogencontaining stream, separating the air stream into high oxygen and lowoxygen content streams, and utilizing the high oxygen content stream ina partial oxidation reaction with natural gas to create a synthesis gascontaining H₂, CO and CO₂. The Starchem Patents are incorporated hereinby reference in their entirety for all purposes.

FIG. 1 discloses the present invention in a simple form. Synthesis gasstream 212, which contains at least about 6 mole % nitrogen, is combinedwith a hydrogen-rich gas stream 214 to form a make up gas (“MUG”) stream216, which has a stoichiometric number greater than 2.05. N_(s) is usedas a measure of how the syn gas will perform in the methanol loop. Assuch, it is commonly treated as a property of the syn gas. A numbergreater than 2 indicates excess hydrogen and a number less than 2indicates a hydrogen deficiency. Excess hydrogen in the methanol loopincreases the reaction rate and concurrently reduces the rate offormation of byproducts. It should be noted, however, that the synthesisgas stream may be produced by any means in accordance with theinvention.

In a specific embodiment of the invention, the synthesis gas stream 212contains at least about 8 mole % nitrogen. In another specificembodiment of the invention, the synthesis gas stream 212 contains atleast about 20 mole % nitrogen. In a more specific embodiment of theinvention, the synthesis gas stream 212 contains at least about 35 mole% nitrogen.

The makeup gas stream 216 is further combined with a recycle gas stream218 to produce a reactor feed stream 220. Feed stream 220 is introducedinto reactor vessel 222, which contains a methanol conversion catalyst.

A portion of the reactor feed stream 220 is then converted intomethanol. The reacted gas stream containing methanol 224 is withdrawnfrom the reactor system and separated into a crude liquid methanolproduct stream 228 and gas stream 230. The gas stream 230 is then splitinto two streams, the first stream being the recycle gas stream 218,which is combined with the makeup gas stream 216 to form the reactorfeed stream 220. The second portion is a purge gas stream 236. The purgegas stream 236 is separated into two streams, the fuel gas stream 234,which contains CH₄, CO, and CO₂ and N₂, and a hydrogen-rich gas stream214. The hydrogen-rich gas stream 214 is then mixed into the synthesisgas stream 212 to form the makeup gas stream 216.

FIG. 2 discloses a specific embodiment of the invention. The processbegins by providing an air stream 200. Air stream 200 is preferably acompressed air stream having an oxygen content of less than about 22%.Air stream 200 is introduced into a separation system, which yields anenriched oxygen stream 206 comprising at least about 28% oxygen. In oneembodiment of the invention, the oxygen content of the air stream isenhanced to between about 35% to about 50%, thereby creating an enhancedoxygen stream. In a specific embodiment, enriched oxygen stream 206contains at least about 42% oxygen. In another specific embodiment, theenriched oxygen stream 206 has an oxygen content of approximately 70%.In a further embodiment of the invention, the oxygen content of the airstream is enhanced to between about 28% to about 70%, thereby creatingan enhanced oxygen stream. The separation system may comprise asemi-permeable membrane, a PSA, or other similar system. Alternatively,the air stream may be enhanced by mixing in oxygen created from a C-ASUunit, or from other known oxygen purification systems, to produceenriched air 206. It should be noted, however, that the synthesis gasstream may be produced by any means in accordance with the invention.

A natural gas stream containing methane 210 is partially oxidized in anATR using an enhanced oxygen stream 206 to create synthesis gas stream212. In one embodiment of the invention, the partial oxidizing stepcreates a synthesis gas stream 212 containing H₂, CO, and CO₂ in astoichiometric number of (H₂−CO₂)/(CO₂+CO), which is less than about1.80; i.e., it is deficient in hydrogen. In a specific embodiment of theinvention, the synthesis gas stream 212 is created that has astoichiometric number of about 1.77. In another specific embodiment ofthe invention, a synthesis gas 212 is created that has a stoichiometricnumber of about 1.73. In yet another specific embodiment of theinvention, a synthesis gas stream is formed that has a number in therange of about 1.34 and about 1.80.

In a further embodiment of the invention, when hydrogen is extractedfrom the purge gas stream 236 to form a hydrogen rich stream 214 that iscombined with hydrogen poor synthesis gas 212 and recycle gas 218, areactor feed gas 220 is produced in the reactor 222 with astoichiometric number between about 2.05 and about 10.

A portion of the reactor feed stream is converted into methanol. Thereacted gas 224, which contains methanol, is withdrawn from the reactorvessel 222 and separated into crude liquid methanol product stream 228and gas stream 230. The gas stream 230 is then split into two streams,the first stream being recycle gas stream 218, which is combined withthe makeup gas stream 216 to form the reactor feed gas stream 220. Thesecond portion is a purge gas stream 236. The purge gas stream 236 isseparated into two streams, the fuel gas stream 234, which contains CH₄,CO, and CO₂ and N₂, and a hydrogen-rich gas stream 214. Thehydrogen-rich stream 214 is then mixed into the synthesis gas stream.

Using the synthesis gas production method disclosed in FIG. 2, thesynthesis gas stream will generally comprise approximately 19% nitrogen.In some embodiments, however, the nitrogen content will be in the rangeof about 6% to about 50%. The present invention permits extremely highlevels of inerts to be carried into the methanol loop contrary to 80years of industry practice. The large quantity of reactants in thesignificantly increased purge stream are separated from the inertportion of the purge stream and returned to be mixed with the incomingsyn gas. This allows a large purge without the loss of valuablereactants. Among other processes, the separation can be done bymembranes or PSA.

As discussed above, a method has been previously disclosed whereinmethanol is produced by contacting a synthesis gas formed from anoxygen-enriched gas stream for the partial oxidation of natural gasusing a methanol loop reactor system. There are key distinctions,however, between this prior art system and the invention described andclaimed herein. In the prior art system, a portion of the synthesis gasis not subjected to the methanol production synthesis loop. Rather, thesynthesis gas is split into two distinct streams on leaving the ATR. Thefirst stream is diverted to a water gas shift reactor, converting CO toH₂, and then through a pressure swing absorber to extract the H₂. The H₂that has been distracted joins the second stream and the combined flowis a hydrogen enhanced syn gas with a N_(s) equal to about 1.85. Sincethe extraction of the H₂ requires the use of additional equipment, oneskilled in the art will recognize that the elimination of thisextraction step results in a methanol production process that is moreefficient and more economical. Additionally, the total methanol yieldfrom the AUS process is lowered by the additional purge required as aresult of the additional process equipment.

In a specific embodiment of the present invention, the syn gas comprisesabout 19 mole % nitrogen and the methanol loop is operated with a highlevel of H₂. The reactor feed gas, therefore, has a stoichiometricnumber of about 3.3. The excess H₂ reacts to maintain the CO at a verylow level, i.e., about 3.8 mol %. Thus, there is very little CO in thepurge gas. This enables the mols of CO in the fuel gas to be maintainedat a value less than 10% of the CO entering the system in the synthesisgas. Similarly, the H₂ is separated from the purge stream leaving behindin the fuel gas an amount of H₂ that is less than 3.5% of the H₂entering the system in the synthesis gas. The sum of the mols ofreactants (H₂+CO) leaving the system in the fuel gas is about 5% of thatentering the system as syn gas. The CO₂ that has not reacted with H₂ inthe methanol loop will leave the system in the fuel gas. This amounts toabout 15% of the CO₂ entering the system in the synthesis gas. Theretention of H₂ in the methanol loop, reacting the CO to low values andrejecting carbon from the system as CO₂, will always result in a MUGstoichiometric number greater than 2.05.

A prime benefit of the use of air separation systems is that they allowan alternative to cryogenic oxygen, which significantly reduces thecosts associated with the preparation of the synthesis gases. Thepresence of cryogenic oxygen along with large quantities of methanolcreates an extremely hazardous and potentially explosive situation,particularly when there is limited space available, such as on a seagoing vessel. The present invention avoids this hazard and allows marineapplication. All of the embodiments of the invention may be practiced ona seagoing vessel in accordance with the invention. The process can beimplemented and completely contained on a ship, barge, or other seagoingvessel. As such, the invention can be brought to natural gas productionareas, such as those at offshore production facilities, to convert theabundant natural gas into methanol. The methanol can then beperiodically transferred from its seagoing vessel to tankers fortransportation to the market. The nominal capacity of the methods of thepresent invention is approximately 5000 to 15000 metric tons per day(“MTPD”).

The N_(s) the MUG stream can be controlled by the methanol loop designparameters. When the loop pressure is held constant by a back pressurecontroller in the fuel gas stream or varying the power to the MUGcompressor, the inerts will leave the system in the fuel gas stream. Inaddition, there will be some H₂ and CO and a major amount of CO₂ leavingin the fuel gas.

In one embodiment of the invention, the MUG comprises H₂, CO and CO₂ ina ratio of (H₂−CO₂)/(CO₂+CO), which is at least about 2.05. In anotherembodiment of the invention, the MUG comprises H₂, CO, and CO₂ in aratio of (H₂−CO₂)/(CO₂+CO), which is at least about 2.4. In yet anotherembodiment of the invention, the MUG comprises H₂, CO, and CO₂ in aratio of (H₂−CO₂)/(CO₂+CO), which is at least about 3.6.

Although the invention has been described with reference to its variousembodiments, from this description, those skilled in the art mayappreciate changes and modifications thereto, which do not depart fromthe scope and spirit of the invention as described herein and claimedhereafter. The following Examples illustrate certain embodiments of theinvention, in comparison to the AUS Process. The Examples illustratespecific embodiments of the invention, and is not meant to limit thescope of the invention in any way.

Example I

The impact of the amount of nitrogen in the methanol loop in the methodsof the invention on the amount of methanol produced per pound of naturalgas was calculated. With reference to Table 1, below, the calculationwas performed assuming a concentration of 42% oxygen, 21% oxygen and 70%oxygen in the air stream shown in FIG. 2. The ATR temperature wasmaintained at 1820° F. The results are shown below in Table 1.

TABLE 1 STOICHIOMETRIC NUMBERS (N_(s)) AND NITROGEN CONTENT (N₂) IN THEMETHANOL LOOP 42% O₂ Air 21% O₂ Air 70% O₂ Air N_(S) N₂ N_(S) N₂ N_(S)N₂ ATR out 1.73 16.4 Mol % 1.62 37.6 Mol % 1.77 6.12 Mol % Syn gas 1.7320.5 1.62 38.6 1.77 7.57 MUG 2.44 17.3 3.65 23.2 2.05 7.39 RX in 3.3330.5 4.23 27.1 2.53 21.7 RX out 4.16 34.2 5.09 28.9 2.85 25.2 Purge gas4.24 36.9 5.06 31.1 2.94 27.8 H₂ Rich gas 7.81 8.5 6.79 7.6 5.32 6.4Fuel Gas 0.27 76.3 0.59 80.2 0.12 60.0 Mol % (H₂ + CO) 5.5 15.0 2.7 Lostin Fuel Gas Mol % CO₂ 15.7 50.6 10.1 Lost in Fuel Gas Methanol/ 1.571.37 1.61 Natural Gas Pound/pound

The results show the level of oxygen enrichment has an insignificanteffect on the stoichiometric number of the effluent from the ATR. It isless than 1.77. Table 1 demonstrates that the process shown in FIG. 1will produce a MUG with stoichiometric number greater than about 2.05when supplied with syn gas containing nitrogen greater than about 7.6mol %. When the nitrogen is discharged from the methanol loop, excesscarbon in the form of CO₂ is discharged along with it leaving thehydrogen in surplus in the methanol loop.

The methanol made per pound of natural gas increased 15% when the oxygenconcentration increased from 21% to 42%. However, the increase inmethanol was only 2.5% when the oxygen increased from 42% to 70%.

Example II

With reference to FIGS. 3 and 4 the methanol production processdescribed in the AUS Process was used to produce methanol using an Aspenprocess simulator. Aspen standard data was used, including equilibriumconstants out of the ATR and the methanol reactors (35° F. approach), aswell as membrane and PSA efficiencies.

Parameters for the study were from the Starchem Plantship analysisperformed by Lurgi. The parameters dictated the natural gas and air flowrates, steam/carbon, pressures, cooling water temperatures, etc. A waterquench to 500° F. out of the ATR was used when flow was into path A.Quench is necessary to avoid metal dusting and provide steam for thewater gas shift. For calculations with zero flow into path A, also knownas the H₂ extraction stream, no quench was used.

Two sessions were performed. The first involved varying the percentageof flow into path A in 10% increments. It was found that any amount offlow into path A resulted in a decreasing methanol yield as the flowincreased. The second session involved increasing the efficiency of H₂separation in path A from 70% to 85%.

The results from the second session are shown in FIG. 4. The syn gas,after combining with stream A, had a stoichiometric number of 1.85 andthe MUG stoichiometric number was 2.8. The production of methanoldecreased as flow was increased into path A. As illustrated, oneembodiment of the processes of the invention, i.e. zero flow into pathA, produced more methanol than the AUS Process.

The total amount of inerts that must be purged from the process is thesame in both the AUS Process and the methods of the invention describeherein. The purge from path A, however, always has a higherconcentration of reactants (H₂+CO) than the purge from the methanolloop. Thus, more reactants that could be converted to methanol are lostwhen there is flow into path A. When path A is utilized, methanol isproduced at a rate of 997,948 lb/hr. As the amount of synthesis gasdiverted to path A is increased, however, the amount of methanolproduced decreases. When path A diverts 30% of the synthesis gas,methanol is produced at a rate of only 942,306 lb/hr., which is an atleast 5% decrease.

The method of control of the invention described and claimed hereinrenders the AUS Process split of synthesis gas into two streams totallyunnecessary and, in fact, harmful to the methanol yield. One stream inAUS Process has the hydrogen content enhanced by water gas shift andrejects nitrogen and CO₂. In the present methods of the invention, thewater gas shift occurs in the methanol reactor and the N₂ and CO₂rejection are performed at a much lower cost and at the same time, moreof the reactants are preserved to make additional methanol.

Although the invention has been described with reference to its variousembodiments, from this description, those skilled in the art willunderstand that certain changes and modifications to the variousembodiments of the invention, which do not depart from the scope andspirit of the invention, are nevertheless within the scope of theinvention.

1. A process for producing methanol, comprising the steps of: providingan air stream comprising oxygen; providing a natural gas streamcomprising methane; partially oxidizing the natural gas stream in anautothermal reformer using the air stream to create a synthesis gasstream comprising H₂, CO, and CO₂ and N₂; combining the synthesis gasstream with a hydrogen-rich gas stream to form a makeup gas stream;combining the makeup gas stream with a recycle gas stream to produce areactor feed stream; introducing the reactor feed stream into a reactorsystem containing a methanol conversion catalyst; wherein a portion ofthe reactor feed stream is converted to methanol; withdrawing a reactedgas stream from the reactor system; separating the reacted gas streaminto a crude methanol product stream and a gas stream; splitting the gasstream into a recycle gas stream and a purge gas stream; combining therecycle gas stream with the makeup gas stream to form the reactor feedstream; separating the purge gas stream into a fuel gas streamcomprising CH₄, and N₂ and a hydrogen-rich stream comprising H₂; andmixing the hydrogen-rich stream into the synthesis gas stream.
 2. Theprocess of claim 1 wherein said air stream has an oxygen content lessthan 22%, and said process further includes the step of enhancing theoxygen content of the air stream to between about 28% and 94% oxygen. 3.The process of claim 2 wherein the oxygen content of the air stream isenhanced to between about 28% and 70%.
 4. The process of claim 2 whereinthe oxygen content of the air stream is enhanced to between about 35%and 50%.
 5. The process of claim 1 wherein the synthesis gas streamcomprises at least about 6 mole % N₂.
 6. The process of claim 1 whereinthe synthesis gas stream comprises at least about 8 mole % N₂.
 7. Theprocess of claim 1 wherein the synthesis gas stream comprises at leastabout 20 mole % N₂.
 8. The process of claim 1 wherein the synthesis gasstream comprises at least about 35 mole % N₂.
 9. The process of claim 1wherein the natural gas is converted to methanol on a seagoing vessel.10. The process of claim 1 wherein the synthesis gas stream comprisesH₂, CO, and CO₂ in a ratio of (H₂−CO₂)/(CO₂+CO), which is less thanabout 1.77.
 11. The process of claim 1 wherein the synthesis gas streamcomprises H₂, CO, and CO₂ in a ratio of (H₂−CO₂)/(CO₂+CO), which is lessthan about 1.73.
 12. The process of claim 1 wherein the synthesis gasstream comprises H₂, CO, and CO₂ in a ratio of (H₂−CO₂)/(CO₂+CO), whichis between about 1.34 and about 1.80.
 13. The process of claim 1 whereinthe makeup gas stream comprises H₂, CO, and CO₂ in a ratio of(H₂−CO₂)/(CO₂+CO), which is at least about 2.05.
 14. The process ofclaim 1 wherein the makeup gas stream comprises H₂, CO, and CO₂ in aratio of (H₂−CO₂)/(CO₂+CO), which is at least about 2.4.
 15. The processof claim 1 wherein the makeup gas stream comprises H₂, CO, and CO₂ in aratio of (H₂−CO₂)/(CO₂+CO), which is at least about 3.6.